Before the answer I owe you an apology - the Mint family is large, and aside from a large number of species, each has a number of varieties. Different varieties of the same species might have similar chemical composition and pharmacological effects, but look a bit different. The fact is, that I'm not 100% sure from your photo that it is Mentha x piperita that we are talking about (most of my sources state that it has pink flowers; leaves are a shaped a bit differently but this might be because the plant is young); still, it might be. Determining the species is tricky even for professionals in such plant families (I was hoping that you got the name of the plant when you purchased it/got it from someone to plant in your garden).
Nonetheless, I'll try to answer the best I can:
The only contraindications for using Mentha x piperita (Peppermint) leaves or Mentha arvensis pipericans (Japanese Mint) are gallstones, gallbladder obstruction or inflammation - because most members of Mint family have a cholagogic effect (stimulate bile production and excretion) so the patient might experience colic if they use mint leaves, and medical supervision of such use is advisable.
For Peppermint the average daily dosage is 3-6 g/day (PDR) or 4.5 - 9g of the herbal substance, for preparation of herbal tea, divided in three doses (HMPC, EMeA).
Another contraindication listed for Peppermint in monograph at EMeA is heartburn (gastro-oesophageal reflux) because the condition might worsen with the use of peppermint.
Other from these, there are no known health risks associated with the use of these species, in recommended daily doses.
For: Mentha longifolia (English horsemint), Mentha spicata (Spearmint), Mentha aquatica (Wild mint), PDR states:
No health hazards or side effects are known in conjunction
with the proper administration of designated therapeutic
However, dosages are listed only for M. aquatica as one wineglass per day of infusion prepared from 30g of leaves and 500 ml of water.
Mentha pulegium (Pennyroyal) is associated with hepatotoxicity (prolonged use can damage the liver). However, the plant in your photo doesn't look like M. pulegium to me.
I' recommend taking a sample of your herb to a local botanical garden if possible and asking for help with species determination there, just to be on the safe side. A somewhat less reliable option would be to post the photo of the plant on biology SE and see what they think of it (but determining the species of the plant from a photo is less reliable than with an actual sample).
As for the quantity - I wasn't able to find a reliable source for the number of leaves used, so the safest method might be to measure the quantity you use and see if it fits the recommended doses (or to measure the maximal daily dose and try not to exceed it).
- PDR for Herbal Medicines
- European Medicines Agency Evaluation of Medicines for Human Use - COMMITTEE ON HERBAL MEDICINAL PRODUCTS (HMPC): COMMUNITY HERBAL MONOGRAPH ON MENTHA X PIPERITA L., FOLIUM
- WHO Monographs on Selected Medicinal Plants Volume 2: Folium Menthae Piperitae